This invention relates to coating of the interior walls of tubing constituted of plastic or other dielectric material, and, more particularly, to improved methods and apparatus for depositing a thin but substantially uniform and adherent polymeric coating on the interior surfaces of such tubing, and the coated tubing thereby produced.
Plastic tubing, most particularly that constituted of silicone rubber, is used as a blood conduit in various applications such as blood dialysis units and heart/lung machines utilized in open-heart surgery. In such applications, problems may potentially arise due to interactions between the blood and the plastic tubing wall. Some types of tubing, such as, for example, polyvinyl chloride, typically contain processing aids or other components which are susceptible to being leached from the tube wall into the blood stream, with potential adverse physiological effects on a patient to whose system the blood is delivered through the tubing. Additionally, or alternatively, certain plastic tubing materials such as silicone rubber may absorb or "imbibe" components of the blood into the tubing wall. These phenomena can present problems not only in blood transmission but also where tubing is used as a conduit for other materials, for example, glucose or physiological saline solution for intravenous administration.
The problems of leaching and imbibition can be potentially eliminated by the application of a barrier coating on the inside surface of the tubing. However, there are significant technical obstacles to applying a coating having the combination of properties desirable for a barrier coating over the inside tubing surface. Such a coating must be very thin and preferably conform closely to the macroscopic topography of the tubing surface. It should adhere tightly and reliably to the substrate material, and be highly flexible and tough so that it does not limit the flexibility of the tubing.
Preservation of the macroscopic topography of the inside tubing surface is particularly important in the practical evaluation of surface/blood interaction. Certain methods of surface modification such as graft copolymerization, which may otherwise be effective for providing a barrier layer, tend to alter surface topography as well as surface chemical properties. As a result, it becomes difficult to separate the influence of chemical modification from that of physical modification in evaluating the effect on blood/surface interaction.
Blood/surface interactions may limit the suitability of a particular tubing for use as a conduit for blood. Thus, for example, tubing that is knitted or woven from synthetic polymeric fibers is conventionally used as a vascular prosthesis for replacement of large arteries, such as the aorta. After implantation, tissue growth through the woven structure provides a natural surface over which the blood flows. However, such knitted or woven tubing is generally not suitable for replacement of smaller vessels, since tissue ingrowth or thrombogenic reactions may cause it to become obstructed. Certain plastic materials may be suitable as an athrombogenic inner coating on a knitted prosthesis, but problems of leaching and imbibition need also be addressed.
Attempts have been made to provide barrier coatings for the interior of plastic tubing by depositing a polymer coating produced by glow discharge (plasma) polymerization on the inside tube surface. However, using conventional glow discharge polymerization apparatus, it has been found to be almost impossible to uniformly coat the inner surface of small diameter plastic tubing. For example, when tubing 3 mm to 6 mm I.D., having a length to diameter ratio of 100 or greater, is placed in a large plasma reactor, the plasma does not penetrate inside the tubing Instead, the plasma is quenched near the ends of the tubing and only small portions of the interior surface near the ends become coated. By utilizing a small glass tube reactor, it has been found that the plasm may be forced to penetrate into the interior of the tubing, thereby making it possible to provide a coating on the inside wall of tubing having a length of one meter or longer. However, the coating produced in this manner is found to vary along the length of the tubing with respect to both thickness and chemical nature of the plasma polymer. This results from the inherent difficulty of providing an even supply of monomer to all of the surface to be coated, and from the relative location of the glow discharge in the monomer flow. See, H. Yasuda and T. Hirotsu, J. Polym. Sci., Polym. Chem. Ed., 16, 229 (1978); H. Yasuda and T. Hirotsu, J. Polym. Sci., Polym. Chem. Ed., 16, 313 (1978); and, H. Yasuda and N. Morosoff, J. Appl. Polym. Sci., 23, 1003 (1979). Although glow generally extends along a considerable length of tubing, most polymerization occurs at the tip of the glow against monomer flow, and not enough monomer can be supplied to the downstream portion of the tube. Thus, monomer consumption and the extent of polymer deposit varies significantly along the tubing length.
Other methods of depositing a very thin polymeric coating generally lead to the formation of a spotty deposit containing significant areas which are uncoated, so that in the case of coated tubing, the substrate surface is exposed to the fluids for which the tubing serves as a conduit. Accordingly, a need has remained in the art for a method for depositing a thin, substantially uniform coating that is free from defects or apertures on the inside surface of the tubing.